The present invention relates to digital bandpass filters and more particularly to a bandpass filter for extracting chrominance signal from a sampled data TV video signal. The filter is designed to perform digital computations at less than the input signal sample rate by providing a decimated filter chrominance signal relative to the input samples.
In digitally processing TV signals it is frequently required to separate the chrominance and luminance components of the composite TV signal to perform selective processing of the two components. The separation process is normally accomplished by comb filtering the composite signal. In the digital domain it is advantageous to comb filter the signal by selectively combing that part of the composite video signal frequency spectrum containing the chrominance signal component to produce a combined chrominance signal then subtracting the combed chrominance signal from the original composite signal to produce a combed luminance signal.
The chrominance spectral regions of the composite signal is obtained by digitally bandpass filtering the composite signal in a linear phase FIR filter. The sample rate of a digital composite TV signal is at least three times the chrominance subcarrier frequency f.sub.sc, to satisfy the Nyquist sampling criterion. However, it has been determined, for systems considerations to be more advantageous to use a four times subcarrier sample frequency which for the NTSC system relates to a sample rate of 14.3181818 MHz. It will readily be appreciated by those skilled in the art of FIR digital filter design that a 14.3181818 MHz sample rate imposes severe design constraints on a real time processing FIR digital filter.
The unique nature of composite TV signals however, permits filtering the chrominance signal at less than the composite video sample rate. The composite signal is the sum of a wideband luminance signal and a narrower band subcarrier modulated in quadrature by I and Q color signals.
If the composite video is sampled and digitized by a 4f.sub.sc sampling circuit with successive sampling points having 57 degree, 147 degree, 237 degree and 327 degree phase relationship to the chroma burst the sequence of samples will correspond to +I, +Q, -I and -Q related signals. The -I and -Q samples impart substantially no additional chrominance information over that provided by the +I and +Q samples and in general need not be processed. Thus to extract the chrominance information from a digital composite video signal with signal samples occurring at a 4f.sub.sc rate the filter only has to produce filtered +I and +Q signal samples. As such the number of computations performed by the filter is substantially reduced. A complete +I, +Q, -I and -Q filtered signal may thereafter be recreated, if desired, by interpolating successive filtered +I samples and successive +Q samples, complementing and time division multiplexing the interpolated samples with the filtered samples in the appropriate sequence.